An Internet Service Provider (ISP) is the industry term for the company that is able to provide you with access to the Internet, typically from a computer. Your ISP makes the Internet a possibility. In other words, you can have a computer with a built-in modem and could have a router for networking, but without a subscription with an ISP, you won't have a connection to the Internet.
An ISP is your gateway to the Internet and everything else you can do online. The second your connection is activated and set up, you'll be able to send emails, go shopping, do research and more. The ISP is the link or conduit between your computer and all the other "servers" on the Internet.
Every home or organisation with Internet access has an ISP. The good news is, we don't all have to have the same provider to communicate with each other and we don't have to pay anything extra to communicate with someone who has a different ISP.